1. Field of the Invention
This invention relates to light-emitting diode (LED) lamps and more particularly to a white phosphor-coated LED lamp with tunable correlated color temperatures along the Planckian locus in the chromaticity diagram.
2. Description of the Related Art
Solid-state lighting (SSL) from semiconductor light-emitting diodes (LEDs) has received much attention in general lighting applications today. Because of its potential for more energy savings, better environmental protection (more eco-friendly, no mercury used, and no UV and infrared light emission), higher efficiency, smaller size, and much longer lifetime than conventional incandescent bulbs and fluorescent tubes, the LED-based solid-state lighting will be a mainstream for general lighting in the near future. Meanwhile, as LED technologies develop with the drive for energy efficiency and clean technologies worldwide, more families and organizations will adopt LED lighting for their illumination applications. For this trend, the Energy Star program specifies in CIE 1931 chromaticity diagram the range of chromaticities of white light recommended for general lighting with solid state lighting (SSL) products.
According to the CIE colorimetric system, a chromaticity coordinate (x, y) or (u′, v′) on the 1931 or 1976 chromaticity diagram is usually used to define a color. However, the chromaticity of a white light is more conveniently expressed by a correlated color temperature (CCT) and a distance from the Planckian locus, Duv. Whereas a nominal CCT is used to convey a specification of white light chromaticity for a product, a target CCT represents a value that the product is designed to produce. Although individual samples of the product may deviate from the target CCT due to production variations, they should be controlled to be within a tolerance. According to the Energy Star program, SSL products shall have chromaticity values that fall into one of eight nominal CCT categories, that is, 2700, 3000, 3500, 4000, 4500, 5000, 5700, and 6500 K, consistent with 7-step chromaticity quadrangles and Duv tolerances. In other words, SSL products with a given nominal CCT should have the defined target CCT and Duv, and the values of individual samples should be within the tolerances of the CCT and of the Duv. Two examples are given below. For the nominal CCT of 2700 K, the target CCT and Duv should have their tolerances such as 2725±145K and 0.000±0.006, respectively. For the nominal CCT of 4000 K, the target CCT and Duv should have their tolerances such as 3985±275K and 0.001±0.006, respectively.
To create a white light from LEDs, one may choose either one of two notable approaches—mixing of three or more primary color LEDs such as trichromatic or tetrachromatic RGB (red, green, and blue) LEDs or use of a blue or ultraviolet LED with wavelength down-conversion phosphor so as to have dedicated single color (e.g. warm-white, day-white or cool-white). For the first approach, LEDs with different dominant wavelengths emit narrowband light perceived as different saturated colors with spectral widths ranging from 20 to 35 nm. By using non-imaging optics to mix multicolor cluster of red, green, and blue LEDs with proper dominant wavelengths and proper intensity proportions, a white light with any correlated color temperature can be generated. For RGB color mixing, there are an infinite number of metamers due to metamerism. To generate a white light that meets the Energy Star requirements, accurate additive color mix proportions must be maintained during LED and lamp assembly production. This would involve extensive test for each LED used or introduction of active electronic control circuits to balance the LED output. In this case, the cost will be too high to produce such products economically.
Although a general multichip RGB with proper dominant wavelengths and proper intensity proportions can provide easy color management, it is not easy to stabilize a specific chromaticity over time while LED junction temperatures change from ambient temperature to 120° C. or higher because individual LED exhibits different thermal dependencies. For example, as the junction temperature changes from 20 to 100° C., the intensity can change 60% and 20% for red and amber AlInGaP LEDs and blue InGaN LEDs, respectively. Temperature also affects the peak emission wavelength with a 0.3 to 0.6 nm/° C. drift. Moreover, the LEDs may degrade in brightness and change in color over time. In specific lighting applications, a plurality of LEDs must be used in a lamp to generate enough lumen output. Individual LED used in these LED clusters, however, has different spectral and electrical properties although its nominal characteristics are the same. It is also true that even in a batch of LEDs produced, the optical and electrical properties of LEDs may vary due to defects in the materials and variations in the manufacturing process. Furthermore, the spectral and electrical properties of LEDs are significantly affected by their junction temperatures, which further depend on LED chip design and specifications, and operating conditions. Such variability of the optical and electrical properties can cause different LEDs to deteriorate at different rates. In this case, even a small intensity change that in turn results in a change of the emitted RGB proportions can present perceptible color shifts.
To deal with these thermal issues, one may use optical and thermal feedback or feed-forward circuit to maintain the chromaticity to within one MacAdam ellipse, especially if the luminaire is being dimmed while the LED junction temperatures vary rapidly. Nevertheless, the approach is too expensive to be adopted in practice. It is, therefore, the purpose of the present invention to provide a scheme effectively alleviating such thermal dependence of color shifts.
The second approach in generating a white light involves use of phosphor-coated LEDs (pcLEDs)—blue-emitting InGaN LEDs coated with one or more layers of phosphors such as cerium-doped yttrium aluminum garnet (YAG). The phosphors down-convert a portion of the emitted light to a wideband yellow light which in turn mixes with the primary blue emission to generate a white light perceived as “cool” white with color temperatures ranging from 4500 K to 10000 K. The advantages of phosphor-converted white LEDs include relatively low cost and great color stability over a wide range of temperatures. However, white pcLEDs suffer from a lower efficiency than normal LEDs do on account of the heat loss from the Stokes shift and other deterioration mechanisms of phosphors. Because the design and production of an LED lighting system using such narrowband emitters with phosphor conversion is simpler and less expensive than that of a complex RGB system, the majority of high intensity white pcLED lighting systems today on the market are produced using phosphor conversion.
Conventional white pcLEDs encounter a fundamental trade-off between color rendering index (CRI) and the luminous efficacy. The CRI, determined by spectral power distribution (SPD) of a light source, is a critical characteristic of the light source in general lighting applications. High CRIs generally require a broad emission spectrum distributed throughout the visible region; the sun, blackbody radiation, and almost all incandescent bulbs emit a white light with a CRI of 100. In general, CRI values in the 70s are considered acceptable, whereas the Energy Star program requires integral LED lamps to have a minimum CRI of 80. Currently available warm-white pcLEDs with low color temperatures provide wider SPD and better CRI than cool-white pcLEDs do, but phosphors used in warm-white LEDs are inefficient in providing lumen output in comparison with RGB LED clusters. Therefore, when energy efficiency and high color consistency at low color temperatures are required, LED clusters are recommended. Conversely, when these parameters are less important, or when accurate color rendering is not required, cool-white and warm-white pcLEDs should be adopted. However, if such pcLEDs are mixed with red and green LEDs, efficiency will not decrease even at low color temperatures, taking advantage of higher efficiency for cool-white and RG LEDs than warm-white pcLEDs. This will be discussed in detailed description of the present invention below.
To change color temperatures, one may use a dimmer in an incandescent lamp. When the lamp is dimmed, temperature of its filament decreases. The emitted light looks “warmer”. Further dimmed, the lamp emits light with a color changing from white to yellow, to orange, and to red. Though, the luminous efficacy of the lamp decreases. Most of “white” LEDs are based on blue LEDs with a phosphor coating that generates warm or cool white light. When dimmed, the white light does not appear red but even more bluish. As for white light created by using RGB LED clusters, its color temperature can be modified using different color mixing, but overall LED efficacy decreases with dimming because driver efficiency decreases at low dimming levels.
As LED lighting becomes more popular for home applications, fully integrated LED dimming controls will become a necessity in new houses while LED products need to retrofit and to work with dimmers originally designed for incandescent products. It is, therefore, the purpose of the present invention to use such dimmers only as human interface to control color temperature of the light mixture of cool white light and red and green light, without dimming or changing lumen output of the light.
A prior embodiment of a white light relates to producing nearly achromatic light by additively combining complementary colors from two types of colors of saturated LED sources or their equivalents. It seems that this technique can provide all desired white illuminations in the CCT domain specified in the Energy Star program. In practice, however, this is not the case because red, green, and blue LEDs drift in intensity and wavelength over time and temperature. On the other hand, the simple mixture of two complementary colors or three red, green, and blue colors create a white light with rather poor color rendition. These difficulties render such LED products unsuitable for wide applications.
FIG. 1 is a CIE 1976 UCS chromaticity diagram expressed by (u′, v′) coordinates. FIG. 1 also shows five saturated colors 10, 20, 30, 40, and 50 at dominant wavelengths of 400, 480, 500, 580, and 770 nm, respectively. The eight quadrangles 80 that specify available white color region 70 of SSL are along the Planckian locus 60. Each of the eight quadrangles is defined by the range of CCT and the distance from Planckian locus on the diagram. FIG. 2 is an enlarged view in the white light region with eight quadrangles 11, 12, 13, 14, 15, 16, 17, and 18, representing eight CCT categories at nominal CCTs of 2700, 3000, 3500, 4000, 4500, 5000, 5700, and 6500 K, respectively. The tolerance quadrangles for 2700 K are defined by four (u′, v′) coordinates (0.2666, 0.5384), (0.2535, 0.5325), (0.2573, 0.5155), and (0.2696, 0.5209). Similarly, the tolerance quadrangles for 3000 K are defined by (0.2535, 0.5325), (0.2409, 0.5251), (0.2458, 0.5087), and (0.2573, 0.5155). The tolerance quadrangles for 3500 K are defined by (0.2409, 0.5251), (0.2277, 0.5148), (0.2339, 0.4994), and (0.2458, 0.5087). The tolerance quadrangles for 4000 K are defined by (0.2272, 0.5161), (0.2165, 0.5052), (0.2238, 0.4909), and (0.2334, 0.5007). The tolerance quadrangles for 4500 K are defined by (0.2165, 0.5052), (0.2095, 0.4964), (0.2176, 0.4831), and (0.2238, 0.4909). The tolerance quadrangles for 5000 K are defined by (0.2088, 0.4975), (0.2026, 0.4884), (0.2114, 0.4760), and (0.2169, 0.4842). The tolerance quadrangles for 5700 K are defined by (0.2026, 0.4884), (0.1970, 0.4784), (0.2063, 0.4672), and (0.2114, 0.4760). The tolerance quadrangles for 6500 K are defined by (0.1961, 0.4793), (0.1905, 0.4676), (0.2005, 0.4576), and (0.2055, 0.4682).
Six 7-step MacAdam ellipses 100 overlap the eight quadrangles, showing that nominal CCTs for SSL are consistent with those for fluorescent lamps complying with Energy Star requirements. FIG. 3 illustrates how the additive mixture of light from two LEDs having complementary hues can be combined to form a metameric white light. As shown, the combined beam of two LEDs with complementary hues 110 and 120, one emitting at 493 and the other emitting at 700 nm, respectively, produces a white light 130 located close to CCT of 6504K on the Planckian locus, which is one of standard illuminants, D65, used in CIE colorimetric system. FIG. 3 also depicts a prior art utilizing a combination of two LEDs whose emissions have peak wavelengths 140 and 150 at 505 nm and 615 nm, respectively, to form a white light with a CCT of 2700K near the other standard illuminant A 160 at 2856K. Also shown is a combination of two LEDs whose emissions have peak wavelengths 170 and 180 at 500 nm and 650 nm, respectively, forming a white light 190 with a CCT of 3500K on the Planckian locus. In the same fashion, a combination of two LEDs whose emissions have peak wavelengths from 493 to 505 nm (perceived as green) and from 615 to 700 nm (perceived as red) can cover the entire white light region on the CIE chromaticity diagram.
The drawbacks for this color mixing are two folds: First, because various possible combinations of two LEDs represent a line segment that is substantially perpendicular to the Planckian locus 60, not only wavelength but intensity variations can change coordinates of a resultant color combination such that the resultant coordinate can easily fall outside of white region. Second, the color rendition is poor because there are only two LEDs with narrow spectral width contributing the overall spectral power distribution that is far from that of standard illuminant A or D65.
FIG. 4 is an illustration of a prior art showing color mixing of RGB colors to generate a white light in CIE chromaticity diagram. On the diagram, 210, 220, and 230 represent blue, green, and red colors at wavelengths of 480, 520, and 680 nm, respectively. Area 200 represents chromaticity coordinates of all the possible resultant mixtures of these RGB LED clusters with a contour 205 representing a locus of additive mixtures from these RGB LEDs. As shown, the white light region is small part of this area; any improper combinations of RGB colors due to temperature-dependent intensity fluctuations or wavelength drift will result in a desired chromaticity coordinate out of this white light region. Also shown are three points 240, 250, and 260, representing three possible intermediate wavelengths that can combine 210 at 480 nm to form white lights in the white light region. However, none of the three light mixtures can cover entire white region with Duv within 0.006, meaning that a perceivable color shift occurs.
FIG. 5 is a block diagram using color mixing of RGB LEDs in a prior art. AC or DC power supplies provide power source to the three LED drivers which in turn power red, green, and blue LED arrays with appropriate electric currents based on the control signals sent from a driver controller that determine correct intensity proportions. The light emissions from three LED arrays are mixed using diffuser or mixing optics and thus generate a white light mixture.
To create white light using color mixing and enhance the usage of the yellowish LEDs and red LEDs, Antony Paul Van De Ven, et al. suggests in their patent (U.S. Pat. No. 7,213,940 B1) that two groups of LEDs with different color hues be mixed. As shown in FIG. 6, the first group of LEDs emitting yellowish light has (x, y) color coordinates within an area on the 1931 CIE Chromaticity Diagram defined by points 310, 320, 330, 340 and 350 having coordinates (0.2105, 0.50), (0.1788, 0.5028), (0.1791, 0.5373), (0.2281, 0.5371), and (0.2333, 0.525), respectively. The second group of LEDs emits light having a dominant wavelength in the range from 600 nm (360) to 630 nm (370). Mixing of these two color hues at proper proportions produces a mixture of light having a (u′, v′) coordinate on a 1976 CIE Chromaticity Diagram, which defines a point within MacAdam ellipses of at least one point on the blackbody locus on the Diagram.
As mentioned, LEDs, when operating, intensity fluctuates, and wavelength drifts over time and temperatures. Different LEDs have different drift rates on these two parameters. Therefore, when the two groups of LEDs drift differently, and mixing ratio changes, the (u′, v′) coordinates of the mixture of light may easily shift outside the six MacAdam ellipses on the blackbody locus on the 1976 CIE chromaticity diagram. What is the worst is that the corresponding coordinates of these two groups of LEDs are in the opposite sides of the Planckian locus. The substantial variations inherent to conventional discrete and individual chip LEDs will cause the coordinates of the resultant additive mixture to traverse the u′, v′ chart in a direction generally substantially perpendicular to the Planckian locus into either the yellowish pink (above the Planckian locus) or the yellowish green (below the Planckian locus) region of the u′, v′ diagram.
In many applications of commercial and residential lighting, a white light with reasonably high color fidelity is required. In this area, a white pcLED lamp is used to replace an existing incandescent and halogen bulbs, taking advantages of LED's features. In a floor lighting application, an LED lamp is used to replace a solar light lamp because the latter consumes much power. Use of a high intensity discharge (HID) lamp instead creates much heat and causes the cooling system to consume more energy to cool down the area the lamp located. LED lamps, however, can provide enough lumen output, do not generate heat, and thus are well suited for this application. Both solar light lamps and HID lamps have high color fidelity with color rendering index close to 100 whereas pcLEDs only have a CRI of 70 or less. LED lamps must have improved CRI to justify the replacement, in addition to energy savings. Prior arts that adopt white pcLEDs or RGB LED clusters obviously cannot meet the requirements.